Change IllegalArgumentException to ArgumentError.

pkg/fixnum/int64.dart

Index: pkg/fixnum/int64.dart
diff --git a/pkg/fixnum/int64.dart b/pkg/fixnum/int64.dart
index f54e8df22882be5553027500c93a507e3a6caaab..c617413c4e7d212b361989ead9e1d990f49cd6c8 100644
--- a/pkg/fixnum/int64.dart
+++ b/pkg/fixnum/int64.dart
@@ -420,7 +420,7 @@ class int64 implements intx {
 
   int64 operator <<(int n) {
     if (n < 0) {
-      throw new IllegalArgumentException("$n");
+      throw new ArgumentError("$n");
     }
     n &= 63;
 
@@ -444,7 +444,7 @@ class int64 implements intx {
 
   int64 operator >>(int n) {
     if (n < 0) {
-      throw new IllegalArgumentException("$n");
+      throw new ArgumentError("$n");
     }
     n &= 63;
 
@@ -485,7 +485,7 @@ class int64 implements intx {
 
   int64 shiftRightUnsigned(int n) {
     if (n < 0) {
-      throw new IllegalArgumentException("$n");
+      throw new ArgumentError("$n");
     }
     n &= 63;